Resistive Ram cache to make Flash fly, say boffins

SSDs appeal to ordinary computer users because of their speed and silence. Data centre folk appreciate those qualities too, but also like the SSD's very low power consumption. Energy is no small cost for a data centre, where there can be tens of thousands of drives all slurping electricity at once.

But it's not a free ride. SSDs aren't cheap, and they have longevity issues. Write information on them too many times and you'll only be able to read them after that. The fact that large areas of memory rather than single bits need to be written at a time - and erased first, a process that clears an even bigger space - means they are not well-suited to applications that unceasingly write and re-write data.

Solving these problems is something of a Holy Grail in the storage business. Many options exists - or have, at least, been proposed - but they're all many years from replacing wholesale the NAND Flash chips used in today's SSDs.

One Japanese group, from Chuo University, Tokyo, has suggested a half-way house: using one of the new kinds of non-volatile memory, Resistive Ram (RRam) as a buffer between the outside world and the host drive's NAND chips. This "hybrid" SSD uses Flash for bulk storage, and RRam for speed.

RRam uses a feature of certain dielectric materials, which normally exhibit a high resistance, to spontaneously create physical, permanent paths, called "filaments", down which current can flow. All it takes is the application of sufficiently high but not impractical voltage.

The pathways reduce the material's resistance, but it's possible to break them, returning the dielectric to its high resistance state. Re-apply the voltage, new paths form, and the material re-enters low-resistance mode.

It's this ability to maintain either of two states - high and low resistance - that allows the dielectric to store binary information. Hook it up to a controlling transistor and you have a usable non-volatile memory cell.

And it's one that can switch much more quickly than its Flash equivalents can: 10ns compared to 100,000ns. The upshot: its write speed is much, much higher - closer, in fact, to volatile memory like Ram.

Coupling it with Flash puts the RRam in the role of a cache. Controller algorithms store frequently read data in the RRam, from where the information can be gathered more quickly.

Cache in hand

In the other direction, the RRam cache is large enough so that data can be held there until there's a sufficient quantity of it to write it out to Flash efficiently.

So quite apart from allowing the drive to appear to operate at RRam rather than Flash speeds, this has the added benefit of organising the data to minimise and possibly eliminate the small, random writes, and the data fragmentation they engender, which are the bugbear of Flash performance and reduce chips' longevity.

The design of Flash chips makes it necessary to write a whole chunk of data - called a Page - of up to 4KB in size even if only a few bytes need to be changed. This is because Flash Pages need to be erased before they can be written to. Worse, Flash has to be erased a Block at a time, and a Block is a much larger amount of memory than a Page, 16KB or bigger. Each erase and write operation gobbles up power.

Caching allows that entire 4KB write to comprise new data. So while 4KB still needs to be written, it does so once rather than every time one part of the page changes.

The Chuo University university designed such a hybrid SSD using 256GB of Flash and 1GB of RRam. The team didn't build such a device, but simulations yielded an 11x increase in write performance - 4.2MB/s for the Flash alone, rising to 46MB/s with the RRam cache in place - and a 79 per cent reduction in the energy consumed for all these writes: 0.12J/MB down to 0.024J/MB.

The team reckon that with smarter, 3D chip construction, connecting the controller, RRam and Flash chips with lines that run through each part - called Through Silicon Vias - the energy saving is even greater, with consumption falling to 0.008J/MB.

Reduced writing means increased longevity: the Flash chips in the Hybrid SSD would last, the team reckons, more than seven times as long as those in a Flash-only SSD. Hybrid drives will cost more, of course, but as long as the extra is less than the price of seven SSDs, that's good news for data centres bulk-buying solid-state drives.

A more logical approach would be to migrate to RRam SSDs, but these are some way off. RRam chips aren't expected to go into mass production until 2014. It will be some years more before they reach today's NAND Flash chip prices.

And the Chuo team's approach applies equally to SSDs with a cache of regular Ram. The downside here, of course, is that Ram loses its data when the power is cut, so the drive needs to be able to hold sufficient energy to write the contents of the Ram cache to Flash as soon as a power cut is detected.

While the Japanese team waits for RRam to become more readily available, companies like Sandforce and Indilinx are building better Flash controllers more able to work around NAND's limitations. But RRam's speed advantage will be hard to beat. ®